The Offshore Voyaging Reference Site

Is Boat Electric Drive Green or Greenwashing?

Here at AAC we are all over anything that will reduce carbon emissions, but we also don’t like the pretengineering so prevalent in the electric drive business.

Login to continue reading (scroll down)

Inline Feedbacks
View all comments
Thomas Ray

Using hydrotreated vegetable oil (HVO) will only end up adding more weight to our boats. A buddy of mine drove an old Mercedes that he powered with HVO. He was hungry all the time, gained weight because he constantly was smelling French fries.

On a serious note, as a live aboard I venture to say my overall environmental impact is lower than when living on land. No cars, no big house to cool or heat and the list goes on.

Iain Dell

I’d be interested to learn what sort of percentage of the contaminants generated in the manufacturing process can realistically be ‘captured’ by the factory and thus removed from the calculations.

Eric Klem

Hi John,

Thanks for posting this, I wasn’t aware of that study. I will try to read the full study when I get a chance. One thing I was glad to see is that they are creating an analytical tool to help people make predictions. One of the first rules of optimization is that you need a reasonably good tool that lets you make trade-offs as these are multi-dimensional problems where you can’t just use a simple hand calc to get an accurate answer.

Unfortunately, the conclusions of this are not that surprising. My back of the envelope calculations suggested that if we could magically ignore weight and cost to replace the diesel in our boat with a battery electric setup, we would be doubling our carbon emissions, not reducing them. But that is because we often do over half of our energy usage for a season in a single straight shot with no charging opportunities.

It took me a while to figure out a term for it but I describe the limitation of battery electrics as being poor at range peaking. Battery electric excels at doing a lot of cycles of similar energy such as a short run ferry might do. What isn’t good is buying a huge bank with a lot of embodied carbon emissions so that you can use the majority of the bank once every few years and otherwise lightly cycle it occasionally. It is going to be interesting to see real longevity of battery banks and that will have a huge impact on this the outcome of these calculations.

I do think that there is an interesting case to be made for plug-in hybrids in certain usage scenarios where there is constant relatively low range required and then an occasional range peak which describes our usage scenario although we keep the boat on a mooring and plugging in is not practical. Of course this assumes that you actually plug the thing in and can deal with the extra weight and size.

The battery supply chain has been getting better and a big breakthrough in batteries combined with continued improvement could change these carbon calculations although the weight issue is going to be a bigger hurdle to overcome.


Matt Marsh

That’s a nice summary, Eric, and I agree you are correct.

Battery-electric marine propulsion is viable, environmentally favourable, and economically acceptable *today* in scenarios with lots of short runs interspersed with charging opportunities. The new Wolfe Island Ferry is battery-electric: 20 minutes underway, dock, a robot arm plugs in the cable, 10 minutes of charging and loading/unloading, then repeat. All day, every day. An increasing number of club racers and daysailers around here use 5 to 10 kWh of LFP batteries with an electric motor, and have no diesel or gas engine at all. They charge on shorepower and that’s enough to get them in or out of harbour, or back to harbour (slowly) if the wind fails. Some European short-haul river/harbour freighters, ferries, tugs, and water taxis are starting to go this way as well. Damen, for example, seems quite confident in the tests they’ve done on their RSD and ASD electric tugboats, which get a 30,000 cycle lifetime (30 years of regular use) from a 2.8 MWh battery bank.

We do not yet have the battery technology to build a long-range battery-electric boat with a useful payload capacity. The cost in batteries (i.e. in raw material, and in weight, and in expense) forces too many other compromises.

It’s hard to put a firm figure on something with so many unknowns and variables. That said, if a calculation of the battery capacity required to achieve the necessary range comes in at more than about 15 kWh per tonne of displacement, or if the energy required to charge that battery for one day’s travel needs to come from somewhere other than one night’s shorepower and one day’s sunshine on the onboard solar panels, then I would say there is a pretty good chance that battery-electric drive is not appropriate.

Eric Klem

Hi Matt,

Your metric of 15 kWh/ ton of boat is an interesting way of thinking about it and makes sense. This is obviously specific to weight and size. Your comment got me thinking that you could also define a metric around total energy cycled through a battery versus its capacity to hit the carbon break even point with diesel. Batteries are around 75 kg CO2/ kWh to produce. Diesel is around 10kg C02/gal and 38kWh/gal and if you assume a propulsion efficiency ratio of 0.3, you get 0.88 kg CO2/kWh for diesel. If we simply compare the carbon emissions from producing batteries to the carbon emissions from burning diesel on an equal energy basis, the break even point occurs when you fully cycle your battery pack 87 times (or use 87 times the packs rated capacity with a higher number of smaller cycles). This ignores a lot of stuff like emissions to produce motors, engines, etc., the carbon associated with the electricity source and all of the end of life emissions. If we go from zero carbon electricity to something like coal, it approximately doubles the number of cycles due to the efficiency ratio of the 2 different energy streams. Applying this to the break-even mileage for cars, you get about the right mileage for people charging on renewable electricity so it seems that the 87 number is reasonable. Note that this only applies to propulsion and not charging out of gear which is much less efficient.

It also illustrates why the car example is so different from the boat example. BEV’s typically have 250 miles of range or so which means that by the time they hit 25k miles, they have 100 cycles behind them and in their lifetime they should hit well over 500 cycles and often 1000+. In a boat that goes out 15 nice summer days a year and is sized for the occasional long trip, they might do 2 or 3 cycles a year and only 30-40 in the battery lifetime which is way less than the 87 break-even calculated above. The notion of electrify everything is unfortunately overly simplistic probably due to the general public’s willingness to understand the nuances but at least the really big things like electric cars, heat pumps, etc. are definitely on the right track and hopefully someone can figure out how to get the smaller things like boats on the right track too.

I wouldn’t get too fixated on the exact number 87 full cycles for break-even with diesel for propulsion number but it is useful to know that it is in that range and not 10 or 500. I am sure someone has created a more refined version of this number somewhere.


Matt Marsh

Thanks for sharing this, John.
I don’t see any surprises here. For a very long time – two or three decades at least – there’s been a gap between public perception of what is “green” or “efficient” tech, and what tech actually achieves those goals. It’s always hugely dependent on the exact use case, a nuance which is usually missing from front-page articles.

The overall conclusions seem very reasonable:

Battery-electric propulsion makes sense when you can recharge often, have short hops between recharges, and have a high duty cycle.

Various forms of hybrid-electric propulsion can be logical if you have high house loads, but don’t tend to make much sense if propulsion is overwhelmingly the dominant energy consumer.

Hydrogen requires very expensive equipment, making it hard to justify on economic grounds. It can make environmental sense if the hydrogen itself is clean – but if you’re just splitting natural gas, discarding the carbon, and trying to store the hydrogen for propulsion use, you are greenwashing in the most roundabout and expensive way possible.

If you need long range under power, there is still no substitute for a diesel engine.

If you use the engine only rarely, then the added cost and environmental impact of a fancy powertrain probably won’t outweigh the lifetime emissions of a diesel engine that’s burning a relatively small quantity of fuel per year.

Bob Hodges

If only the politicians were capable of understanding this type of analysis!

David Leckenby

The discussion here leaves out one very important fact. That is, when technology is being developed to solve a problem it often occurs in incremental steps, and the early steps often do not solve the problem, but they are an essential part of the process. The early BEVs were a good example, hopeless range, heavy expensive batteries with high environmental costs. Using the logic used in many of the comments above, we would have stopped there and declared there was no good alternative to fossil fuels. But thanks to early adopters, and visionary tech nerds, here we are with a solution. I’ve just spent 2 seasons cruising on a fully electric Hanse 315 and I sympathise with those early adopters of electric vehicles. We had to make compromises, but you have to actually get out there and do it so that real world issues can be identified and solved. So while the current electric technology might not reduce emissions in every use case, we still have to keep pushing on. Otherwise the only alternative is to keep burning fossil fuels and hoping a solution pops out of nowhere.

David Branyon

Yes, we need early adopters. But too many early adopters can actually be counterproductive in this situation where electrifying your boat completely causes a large increase in your carbon footprint… for now. And just because something does not sell in widespread fashion does not mean that the research stops. As long as there are reasonable expectations that it will sell in the future or has a profit opportunity, the research will generally continue.

Krist Van Besien

There is an interesting French Catamaran that came on the market recently: The Windelo. It is offered as a pure electric boat, with a genset as backup. They have been buildings boats for a couple of years now, and 7 have so far crossed the Atlantic, often donating all their unused diesel at their destination…
So this shows that it is possible for a cruising yacht. Albeit one that takes advantage of the surface area a catamaran provides by installing a lot of solar.
Windelo also has done a lot to reduce the carbon footprint of sailboat building, which is something worth looking into all of itself.

David Leckenby

We carried a small generator on our electric boat and only used in once in 2 years. We think however that it’s important to have that backup and viewed it in the same way we did our liferaft. Necessary to carry it, but hope you never use it.

David Leckenby

That boat is a Hanse 315 (31.5 feet) with a 4kw Torqeedo motor. Under motor it will do 6 knots in ideal conditions, a little underpowered imo, but it’s reasonable. In our first year we had a 2kw generator and never used it, and since we were tight on space I downsized it to a tiny 1kw model for our second year. At 1kw we do about 3 knots, so we never use the generator while underway. Our plan has always been to keep sufficient battery power to get us out of an emergency situation, and if that happens we would then need to use it at anchor to recharge (if there was no sun). The one time we used it was not actually an emergency, we had to sail about 100nm to meet someone and there was no wind for days 🙁

Krist Van Besien

The Windelo is a fully electric, yes. But they also have a lot of Solar. 5kW in total. Our house in the Swiss mountains is fully electric as well (heating, cooking etc…) and consumes between 15 and 35kWh per day depending on the season.
I hence find it entirely plausible that the Windelo does indeed manage to cover its entire energy needs from its solar array. They interviewed one couple that made a transatlantic crossing, and they said that they arrived with full tanks and full batteries…

Jimmy Cornell had only about 1500W of solar. Maybe that was his issue.

Emile Cantin

I’m very interested in an electric conversion, but for a different reason: My engine is original to the boat and is 44 years old at this point. This means a repower is probably on the horizon, so I have to have a plan for when it inevitably happens.

Also, my background is in computer engineering, so I’m much more in my element troubleshooting a configuration issue in some electronic device than diagnosing injection issues on a diesel engine.

With that said, my main interest in an electric conversion is one of cost: I think I’d be much more confident doing a DIY electric conversion and installing everything myself. If installing a new diesel I’d have to hire someone to do the work. Having a diesel generator is also appealing to me as it’s a “module” that I can much more readily replace or take out of the boat for repairs if needed, since it’s much less intertwined with the rest of the boat’s systems.

I also think such a diesel generator would have a longer-lasting life as it would suffer less of the short start-stop cycles we do when getting in and out of harbor (this part would be handled by the batteries). It’s probably easier to soundproof, too.

I completely agree it would be less efficient than a regular diesel engine when motoring for long stretches, but that’s a cost I’m prepared to accept if I end up with a boat that’s more reliable and more easily repaired (by me!).

Emile Cantin

To be more specific, I was looking at kits from Thunderstruck EV. A basic motor kit (motor & controller) is 3600$ for 18kW, and a 10kW genset is around 10-15k, maybe less if bought used. I think 10kW is probably sufficient to drive at cruise speed, the last 8kW can be drawn from the batteries if we need to “redline the engine” for a short while. Maybe this is the part where I’m completely wrong, though. My boat currently has a 30hp engine and I usually cruise at 6.4kts (I forget the exact RPM, though).

Add a few batteries, odds and ends and I think you end up around 20k total. I’ve made a few calls 2 years ago and was quoted about 25k for a full re-power (the spreadsheet grossly underestimates that number). Of course, I haven’t counted a lot of batteries here, but I’d aim for only about half an hour to an hour of motoring range, well enough time to get out of the marina and hoist the sails if it’s that kind of day.

You say a repower is simpler than it seems, but honestly I have limited skills in that area, and I don’t have much interest in learning them. I can do an oil change, but I’m deathly scared of changing my fuel filter after I messed it up on my previous boat and some air made its way in the fuel line leaving me stranded at a very bad moment (I’ll have to do it very soon, wish me luck!). I’d be much more confident in an electrical installation I can understand and troubleshoot myself. I can, and have, debugged my electronics at sea; I mostly use open-source software that let me peer under the hood which makes it much easier.

Not all of us grew up messing with engines; I actually grew up programming my calculator! I believe that makes me very different from most sailors on this site and “good old mechanical stuff’ is to me the black box that “newfangled electronics” are to others.

Good advice about rebuilding the engine; I’ve looked into it but ultimately decided against, good to know you agree.

Eric Klem

Hi Emille,

Since your concern is mechanical, I would be pretty surprised if it ended up being easier to install a hybrid mechanically than a shaft driven diesel. To expand a bit on John’s earlier thoughts:

The trickiest thing for a DIY’er doing an engine swap is usually the fabrication of new engine beds. For the diesel, if you get a Beta they will make custom mounts for you if you do a bit of measuring, otherwise it is tricky. For the hybrid, you will need to fab up custom motor mounts as well as custom mounts for the genset and both are tricky. That genset is also going to be tricky as the place in the boat with good structural tie-ins is now occupied by the motor so you need to find a new place and fab up a mounting system that structurally ties it in as having one get loose would be a disaster. Aligning the electric motor is probably a bit easier than a diesel but not a whole lot (some people find this super frustrating so find a friend or hire a mechanic to do the alignment and final fire up if needed).

Lifting heavy things can be intimidating but entirely doable, especially if the mast is up but I don’t see a differentiator here.

Most systems will be the same between a diesel prime mover and a diesel genset. Both will have the same fuel, exhaust, cooling water and gauges requirements. The throttle will be different and a bit easier to hook up. The high current wiring will also be a bit different. On their own, each of these systems are reasonably straight forward, what gets hard/intimidating is looking at the whole problem at once. The hybrid system will have its own complexity and usually involves a DIY liquid cooling system, etc. so the above just looks at the diesel end.

The other problem will be space. You might get lucky and be able to put the genset in the same area as the motor but if it doesn’t fit, it will get real hard as you will also have a few big batteries to put in. Keep in mind that the amount of weight and its location is important so it isn’t just a matter of where there physically is space.

I hope this isn’t too negative, just trying to be realistic. The first time I swapped an engine it was really intimidating and I had to just keep plugging ahead and now I don’t worry too much as I know that I can tackle each individual issue and get it done. You may find that if you need to have someone do the diesel for you, you will also need to do the same for large parts of the hybrid system.


Emile Cantin

Hi Eric,

One thing which I haven’t mentioned in this thread yet is the fact that my boat has a V-drive, so the engine is pushed way back under the cockpit sole. This makes modifications to the engine bed pretty intimidating, and also restricts the choices in terms of transmission / engine selection. An electric motor would be able to be installed the normal way though, there’s enough room there for that.

Good point about mounting the genset; it needs to stay put but it doesn’t need to take the full thrust from the prop in both directions. Alignment is also not an issue with the genset. The only critical alignment here is with the electric motor, but being much lighter I think makes it much easier to align.

I know the genset will need a few hoses and whatnot, but messing this up and getting an air bubble in the fuel line will not leave me without propulsion in the middle of a marina (speaking from experience!). There’s also no transmission that can slip or come undone from the engine (again, speaking from experience).

So yeah, I know I’ll end up with mostly the same systems as a whole, but it’s the decoupling of these systems in modular, “easily”-replacable components that appeals to me here.

Eric Klem

Hi Emile,

Yes V-drives can be a bit of a pain, our last 2 boats have had them. Some are add-on units that bolt to the transmission and some are integrated to the transmission. If your boat can fit it, the ones that integrate to the transmission are generally much easier to deal with. For example, the Hurth HBW150 is one of these.

I would be happy to give a few thoughts on sizing but it should probably be over on the hybrid numbers article. I bet others have had experiences on this as well that might be useful.


Emile Cantin

Yes I’m comparing the “clé en main” cost for a diesel to only the parts for a series-hybrid, because I feel I have the ability and the interest to take on such a project myself.

Regarding generators, I was thinking of a Fischer-Panda genset, which looks like a pretty reputable manufacturer. They do DC gensets, which avoids the need for costly AC-DC electronics (or at least let me size these down for a “shore power” scenario).

My main question honestly is the sizing of it, which can easily make of break the financial equation.

Thinking out loud here, my existing engine is 30HP (22,3kW), but that’s at max RPM. I probably only need to size the generator to cruising RPM which is only around 15-18HP if I remember correctly (I need to look at the curves, but the manual is on the boat right now). This leads me to believe a 12-14kW genset might be sufficient to drive the boat at cruising speed, but I’m not really accounting for losses here, and I’m not leaving much headroom.

One thing which I find appealing with a hybrid system on a cruising boat is power management when off the grid. You’re not worried about running the engine at low load if you need to charge the batteries when said engine’s only purpose is exactly that. But that implies a battery bank sized accordingly, which again, might easily tip the financial equation the other way.

One thing is sure, though: I’m not about to rip a perfectly good working engine out of my boat to install this; I’m just doing my homework so I have a plan when I do need to replace the engine.

Dick Stevenson

Hi Emile,
I had a DC genset for decades and believed that, for cruising boats, generating DC directly made a great deal of sense, especially for smaller boats. Still do. However, in much research over decades, there are no DC gensets that check the reliability box. I know of two boats that spent thousands of dollars and months of cruising time fussing with their Fisher Panda DC gensets before throwing them overboard and turning them into moorings. And other stories of troubles.
Those who have made DC gensets work, generally a small Kubota tractor engine attached to a big alternator, are mechanically minded and experienced-with-diesel people who have cobbled together a system.
It should not be that complicated so I suspect that there are no commercially successful forays into this area because it promises such low volume for sales:. But my research indicates that no one has succeeded yet.
My best Dick Stevenson, s/v Alchemy  

Emile Cantin

Good to know, thanks! The specific Fischer-Panda genset I was looking at actually uses a Kubota engine. Is it the engine that was problematic in the 2 boats you mention, or the alternator part?

I’m hoping there will be some development in that area, but I guess using an AC genset might make sense as well. The expensive electronics I was talking about are actually the vehicle side of a J1772 connection, which could come in very handy in the future if marinas install charging stations for electric boats.

Neil McCubbin

Fascinating study, which will hopefully help individuals make good decisions.

Greenest option (no engine) was not evaluated, despite being the one with the most proven experience.
Books on yacht selection I read as a kid included serious discussion on whether having a motor was desirable or not. Early Vendee Globe circumnavigators has no motor, and neither did Captain Cook.

Today, a sailor who wishes to SAIL but needs a motor for marina docking and very limited moving in calm, a small electric motor wit a mix of shore and solar charging will be pretty green. A 1 HP motor will move our 47 ft monohull at 1.5-2 knots in a glassy calm. Cheaper and greener than a diesel. IF we accept the limitations.

Boats are not bought or converted by average people. The decisions are made by each owner for his planned use and preferences
Esch boat owner has to analyse the “greeness” of any propulsion system based on his usage pattern

David Branyon

I want to note the big picture before diving into details. The article states that “Recreational boats account for less than 0.1% of global greenhouse gas (GHG) emissions” and then notes that this is 0.7% of transportation CO2 in the US is from recreational boating.

This is a miniscule amount. And likely >90% of it is from powerboats and probably 50% from outboard engines. My estimate based on these numbers is that well less than 0.01% of global GHG emissions are from sailing vessels. In this case, it would be much better to spend our GHG-reduction resources (time and money) on other, more fruitful pursuits, e.g. our automobiles, houses, etc.

And if you want to grab some low-hanging fruit on your sailboat, run the engine less and don’t run just to generate electricity and don’t have a generator either. Enough solar/wind generation to provide for your electrical needs and an efficient diesel engine for propulsion and propane for cooking is going to be VERY difficult to beat.

Stein Varjord

Hi David,
I have no numbers, but you’re certainly right that sailboats are not a significant contributor to the climate crisis. Still, that’s not a valid argument.

Every person on the planet by him-/herself is a much smaller contributor than the sailboats of the world. We can all correctly say that what we ourselves do or don’t isn’t noticeable when compared to the big numbers change that is needed. It’s totally understandable to think that way, but it’s also totally wrong, of course.

I’m 100% certain that sailboats will mostly have electric propulsion in the near future. The reason will be that owners want to do the right thing, but even more that they want a quiet propulsion that needs almost no maintenance and is almost free to use. This development won’t need any regulations. It’ll happen by itself.

David Branyon

I should clarify: I’m not saying we individually should do nothing because our individual contributions to GHG are insignificant. As you correctly note, it will take significant efforts by a large portion of the billions of earth dwellers if we are to reverse the CO2 trend. I am saying that for almost everyone except a portion of full-time cruisers, there are more beneficial areas of your life to spend your GHG-reduction efforts than your sailboat’s diesel propulsion engine. I want to get the biggest GHG bang for the buck and that is almost always going to be not spending it on the sailboat propulsion engine replacement with an electric motor. Spend that money on solar panels for the house, more efficient appliances, a BEV and charger for the house, drive less, drink tap water instead of bottled, LED lights for your sailboat and other energy consuming redictions for the boat to maximize your impact.

Eric Klem

Hi David,

Totally agree. We live in a limited time/money world so we should prioritize. Every few years we do a rollup of our energy usage. In our case before we started making improvements, heating our house was the biggest thing, fuel for cars was the next and everything else was much lower so we knew where to focus. For people who fly a lot, that can quickly become one of the biggest if not the biggest one. The boat wasn’t completely negligible but it was tiny and one of the harder ones to improve on whereas things like improving a home’s insulation is comparatively easy (but still not easy). If biodiesel or equivalent were available, that would make it easy to make big changes on the boat now but everything else except behavior modification needs large technology improvements.


David Branyon

The quote from Steve Bruce, of ePropulsion:
“The small amount of hours they suggest boats are used for does not correlate with the actual use profiles we are being asked to support.”

This is completely to be expected. People spec boats (and everything else) based on something approximating a worst case scenario, and notoriously overstate their needs. Then what they actually use is significantly less. Steve should know understand this, and probably does.

Brian Russell

If my calculations are incorrect please let me know, but I just don’t see how we can get around the laws of physics. At 1800rpm, our normal motoring rev, my Kubota/Beta 60 ICE is producing about 27kw of energy, according to the factory provided power curve on their data sheet. Over a time period of 1 hour that works out to 27,000 watt-hours of power. At 12v this is equivalent to 2250AH of electricity.(27000/12) Our present battery bank is 850AH ( or 850×12=10,200WH) of AGMs. So If I were to run an electric motor to move our 18T boat, with a modest 500sf of wetted area, at 5.5knots- my run time, assuming a discharge rate of 50%DOD or 425A, would be less than 15 minutes. And converting the system to 48v or whatever doesn’t change the basic calculus. There is a helluva lot of energy stored in that dinosaur juice. I once motored for 92 hours straight in zero wind in the middle of the Atlantic on the way to the Azores, stuck in a huge high that was moving northeast with us. Sure, I could have waited for the wind. But then we would have had to eat the horses…

Eric Klem

Hi Brian,

If you are using the engine power curve, that is the maximum that the engine is capable of at that rpm. However, the actual power output is determined by the propeller curve which is not published as it is boat and prop specific. If the engine outputs more power than the prop can take at that rpm, then the system is imbalanced and will accelerate until the power output and the power used match. My guess is that your real power output at that rpm is less than half of what you stated. The hybrid model John linked does use the prop curve and you can play with it there.

All that said, your general conclusion is still correct. Diesel is incredibly energy dense on a weight and size basis whereas battery are much less so. They are getting better but still have a way to go to support long motoring sessions between charging.


Dick Stevenson

Hi John and all,
Perhaps I missed this notion earlier suggested, but is there a case to be made for an interim possibility/solution before an electric propulsion/diesel free boat is designed?
I am thinking of a boat with a big lithium battery bank and with a diesel generator and a reasonable amount of solar panels. This would have the advantages of an electric drive and of being able to do cooking etc. with electricity.
The solar could cover a percentage of the charging, but never would boat functioning be dependent on solar. The generator would only be run when needed meaning optimal efficiency. For many boats, this would mean there would only be one diesel aboard dispensing with the propulsion diesel and its often-inefficient usage profile. In addition, there is a big weight savings which could go into the battery bank.
And, one would have the advantages of an electric drive which sound not insignificant.
There are many details I have elided, but the idea is there.
The above might be best for bigger boats.
Random thoughts, Dick Stevenson, s/v Alchemy

Eric Klem

Hi Dick,

If the energy comes aboard as diesel and you don’t have an extreme use case like motoring very slowly with a huge engine, the most efficient thing you can do is to simply use it in a well set up diesel drivetrain. What we need to do is to get the energy aboard from much cleaner sources. This usually means electrifying your powertrain as even bad powerplants are way more efficient than a small diesel and the electric drivetrain is pretty efficient. Battery electric works great for frequent users with constant short range requirements and easy charging but long range is not practical right now. Unfortunately, except for very light users, solar just doesn’t bring enough aboard to change the equation.

The one system that I can think of that can work is a plug-in hybrid. This would support a use case of lots of short trips with charging in between and very occasional long range trips. Most travel would be electric from shore power but the boat would not have the range limitations so an occasional long motor would still be possible. I don’t think this is for the long distance voyager but it could work for the very frequent users near us that do a lot of traveling within 30 miles of their home berth but also do a yearly trip that is 200-400 miles away. The whole point of this system is to lower the embodied carbon from the batteries but still get the benefits of being (mostly) electric although infrequent users really have to watch that equation even with smaller banks.


Dick Stevenson

Hi Eric,
Thanks for your thoughts.
Agree, getting energy aboard from cleaner sources is the goal.
Is a “plug-in hybrid” not a fair estimation of what I have proposed: big battery bank to cover most bases and a diesel generator to cover the more demanding times?
For short runs: in and out of anchorages for example, the batteries would power the drive train. And many a day sail might be covered by the charging accomplished when in a marina and shore power has the bank fully charged.
Longer runs (and longer periods at anchor) might be accomplished by running on batteries till reaching a certain depletion level and then running the generator to charge while continuing the run. In this way, longer runs might have the boat running on battery while the generator is used for 15 minutes every hour.
In this way, the diesel would be used in its most efficient manner. The solar would clearly not suffice, but might be a nice addition in much the same way solar augments many vessel’s charging capacities presently.
This proposal gets rid of one diesel (on bigger boats where a generator might be installed) and all the ancillary benefits of getting rid of the propulsion diesel), gets the benefit of an electric drive train. The larger battery bank allows for electric cooking and its benefits as well as the probable elimination of propane.
My best, Dick

Eric Klem

Hi Dick,

I think I misunderstood you as I didn’t see a mention of shore power. Given that you seem to be thinking of that, yes we are talking about something similar.

One potential difference is series vs parallel. I believe you are suggesting a series system. For a boat with very high house loads, this probably pairs best with the plug-in hybrid. With smaller house loads, it would be better to run a clutched parallel where the engine is capable of clutching in and directly driving the prop as that is more efficient than the energy conversions associated with a serial hybrid.

The problem with all of this is that it becomes so use-case dependent. The system could be sized for our use (pretending we gave up our mooring and went to a slip) with say 20 miles of electric only range and then an additional 300 miles of diesel range and I am confident that about half of our miles would be electric. However, if we sold the boat to someone who daysails 20 days a year and motors about 1nm between charging sessions, it would be worse from a carbon perspective. If we sold to a long distance sailor who enjoys rural destinations, the electric system would never get used and it would just be a diesel with a lot of extra weight and embodied carbon. At least a plug-in hybrid is a bit more forgiving of this stuff than a full battery electric but since the boats of most non-full time cruisers see so little use, it is unclear to me how you deal with this.


David Branyon

One more thought to add. There are different “qualities” of energy. Heat is a lower quality and a rotating shaft is a higher quality. By this, I mean that it is very easy to use a rotating shaft to create heat with near 100% conversion efficiency, but it is difficult and much less efficient to rotate a shaft using heat energy. By this definition, electricity is a higher quality energy and diesel fuel is a lower quality.

When we burn diesel fuel in a small engine, we can hope to get some where in the (hopefully upper) 30’s% conversion efficiency (or thermal efficiency). That is ~35% of the fuel energy showing up as rotating shaft power. But when we use fuel to directly heat something (as in cooking), we get very nearly 100% of that fuel energy converted into heating the food.

When you look at this in terms of using renewably generated electricity (i.e. solar, wind, hydro generator), we should use that where we get the biggest bang for the buck. And using it for heat, we get about 1/3 of the reduction in burned fossil fuels as we do if we use it for rotating shaft (e.g. propulsion) power. In other words, it takes 1 kW (okay, maybe 1.1 kW) of electricity to drive a propeller consuming 1 kW. It also takes 1 kW of electricity to heat food at the rate of 1 kW. However, in diesel fuel terms, 1 kW of heating requires about 0.7 L/hr of diesel fuel. (Bear with me, I think the example is easier to understand if we pretend we cook with diesel). Compare this to the amount rate of diesel consumption required to provide 1 kW of shaft power, about 2.0 L/hr.

This makes electric cooking relatively unattractive until we have such a wealth of renewably-generated electricity that we can “afford” to use it in the low payback cooking realm. Worst of all is running a generator to make electricity (at the same ~35% efficiency) and then using that electricity as heat, i.e. cooking. That is just throwing away 65% of the energy in the fuel needlessly, and increasing our carbon output by the same amount.